The present invention relates generally to composite annular structures formed with a metal matrix and with a filament reinforcement. More particularly, it relates to annular structures having a titanium base matrix and reinforced by filaments of silicon carbide and to the HIPing of such structures to enhance the composite character thereof.
The preparation of titanium alloy base foils, sheets, and similar articles and of reinforced structures in which silicon carbide fibers are embedded in a titanium base alloy are described in U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337 assigned to the same assignee as the subject application. The texts of these patents are incorporated herein by reference. Preparation of composites as described in these patents is the subject of intense study inasmuch as the composites have very high strength properties in relation to their weight. One of the properties which is particularly desirable is the high tensile properties imparted to the structures by the high tensile properties of the silicon carbide fibers or filaments. The tensile properties of the structures is related to the rule of mixtures. According to this rule, the proportion of the property, such as tensile property, which is attributed to the filament, as contrasted with the matrix, is determined by the volume percent of the filament present in the structure and by the tensile strength of the filament itself. Similarly the proportion of the same tensile property which is attributed to the matrix is determined by the volume percent of the matrix present in the structure and the tensile strength of the matrix itself.
Prior to the development of the processes described in the above-referenced patents, such structures were prepared by sandwiching the reinforcing filaments between foils of titanium base alloy and pressing the stacks of alternate layers of alloy and reinforcing filament until a composite structure was formed. However, that prior art practice was found to be less than satisfactory when attempts were made to form ring structures in which the filament was an internal reinforcement for the entire ring.
The structures taught in the above-referenced patents and the methods by which they are formed, greatly improved over the earlier practice of forming sandwiches of matrix and reinforcing filament by compression.
Later it was found that while the structures prepared as described in the above-referenced patents have properties which are a great improvement over earlier structures, the attainment of the potentially very high ultimate tensile strength of these structures did not measure up to the values theoretically possible. The testing of composites formed according to the methods taught in the above patents has demonstrated that although modulus values are generally in good agreement with the rule of mixtures predictions, the ultimate tensile strength is usually much lower than predicted by the underlying properties of the individual ingredients to the composite. A number of applications have been filed which are directed toward the overcoming the problem of lower than expected tensile properties. These include an application Ser. No. 07/445,203, filed Dec. 4, 1989 now U.S. Pat. No. 5,201,939 and U.S. Pat. Nos. 4,978,585, issued Dec. 18, 1990; 5,017,438, issued May 21, 1991; and 5,045,407, issued Sep. 3, 1991. The texts of these applications are incorporated herein by reference.
One of the structures which has been found to be particularly desirable in the use of the technology of these reference patents is an annular article having a metal matrix and having silicon carbide filament reinforcement extending many times around the entire ring. Rings of a few inches to a few feet in diameter are prepared with such reinforcing filaments. Such ring structures have very high tensile properties relative to their weight, particularly when compared to structures made entirely of metal.
The fiber reinforced ring can be used, for example, as a reinforcement ring structure for compressor disks of a jet engine. In order to serve to reinforce the disk in a compressor stage of a jet engine a large number of layers of reinforcement are required. It has been found that it is very difficult to continue to add more and more layers of filament reinforcement to a ring structure because of differences in thermal expansion coefficient and other factors.
One of the problems which results from the continued addition of outer layers of filament reinforced matrix to a ring structure is that the outer rings tend to cause a compression and buckling of the outer filaments as conventional consolidation occurs during HIPing. Such buckling of the filaments can cause damage to the filaments and accordingly to the rings of which the filaments are a part. Such buckling tends to occur as the number of fiber layers is increased so that when the number reaches 20 or 30 successive layers, any additional layers which are added beyond such value can result in buckling and damage to the filaments, as the overall structure is consolidated through the HIPing operation.
One way in which this problem has been solved is by forming a series of concentric rings which are then assembled together to provide a reinforced ring structure having more than 100 layers of reinforcement. Such ring structures may be of quite large diameter of the order of a foot or several feet and must nevertheless be nested together within very close tolerances of only a few thousands of an inch.